That's the same Ruggero paper that I meant, so yes I'm familiar with it.

The mechanical data of figure 1A are sliced at a
threshold level of 164 microns/second and
compared with neural threshold data, in figure 3.
They're also sliced and compared at 1.5 nm
displacement. The result is that the neural
response is somewhere between the velocity and
the displacement, but is otherwise about the
same. But you are right that unfortunately, this
single slice does not get at the issue you are
are trying to address.

I do see now that the CF would shift to lower if
it was defined via a sufficiently high threshold
level; such high mechanical thresholds, reached
at around 80 dB SPL, would be hard to compare
with anything neural, since it will be hard to
find a neural rate threshold that will exceeded
at 80 dB for some frequency but not at lower
levels for any frequency (because rate is very
well saturated by that point). So I don't see
how there can be corresponding data of the type
that you suggest the authors have suppressed, or
if it exists it's won't show much interesting in
the region you're referring to due to saturation.

Phase data, on the other hand, do exist and
should be robust, and would be good to look at
for differences. Their Fig. 5, phase data at 75
dB SPL, does not really address the level
dependence, and there's an apparent slope
disagreement that probably comes from assuming a
1 ms delay instead of fitting a delay that makes
the slopes match; Fig. 6 looks at level
dependence, and shows an interesting discrepancy
above 80 dB SPL, in the case a stimulus frequency
far below CF; this is the interesting one to try
to explain, though it says nothing about what
happens within an octave of CF. I don't see
anything about level dependence of neural phase
near CF, which is too bad.

I don't have access to figure 7B of the second
paper; I'm sure it would be fair use for you to
email me a scan of it.

I'm not able to follow your logic in saying that
the non-existence of some figure that you imagine
is evidence of the data relationship that you
claim exists in many publications. Here's a
suggestion: if the data show the effect you are
claiming, interpolate and replot from some data
that do exist, and show us a figure of the sort
you mean, constructed at least approximately from
real data. Then maybe we'll be able to see what
it is that you're seeing in the data, and why you
disagree with Ruggero et al on the interpretation
of their measurements.

Ruggero et al explain that the lack of
understanding of the relationship between neural
and mechanical is due to a dearth of mechanical
data. Now that they've got the mechanical data,
you can't appeal to missing neural data to
support a discrepancy; so show us some data that
illustrates your point.

Dick

At 3:18 PM +0200 10/11/07, Martin Braun wrote:

Richard F. Lyon asked:

Where should I look to study the data on this idea that underlies your
modeling approach?

You can take any data on level dependence of neural responses. None of
them mirrors the half-octave shift of BM tuning.

I've looked at lots of data, and you must be interpreting it differently
from how I am. So if you have something specific that we can look at and
discuss, we can try to resolve that difference. Lacking that, I'll stick
with Ruggero's interpretation that says mechanical and neural are
essentially the same, not different.

OK, here it goes.

1) Mario Ruggero only compared mechanical and neural behavior at threshold.
Had he compared mechanical and neural behavior at high sound levels, he
would have seen the striking dissociation between the two.

2) The mechanical data (basilar membrane BM):
The literature is full of data showing the half octave shift of BM behavior
between low/medium sound levels and high sound levels. A freely available,
and recent, example is this one:

Here we see that, at one single recording location, the BM reacted most
strongly to sound levels of 0, 20, and 40 dB, when the probe tones had a
frequency 9.5 kHz. At a sound level of 100 dB, however, the BM reacted most
strongly, when the probe tones had a frequency 6 kHz.

Interpretation: The low level data reveal the tuning of the outer hair cells
(OHC), whose motility primarily excites the adjacent inner hair cells (IHC)
and secondarily, as a side effect, cause local BM excursions. The high level
data reveal the tuning of the BM proper, the passive BM. The protective
effect of this passive BM tuning: high level sound of 9.5 kHz vibrates the
BM *basalward* of the place of the OHCs tuned to 9.5 kHz. This takes out
energy at the most critical frequency (9.5 kHz) that otherwise might damage
the OHCs.

3) The neural data (auditory nerve fiber):
If neural data would correspond to BM data, the figure above would have a
corresponding figure, where the fiber fired most strongly at sound levels of
0, 20, and 40 dB, when the probe tones had a frequency 9.5 kHz, but most
strongly at sound levels of 100 dB, when the probe tones had a frequency 6
kHz.

Now - and this is the crucial point - such figures do not exist. Did they
exist, you can be certain that Mario Ruggero would have published one,
together with the figure above. The experiments were done, and the figures
were plotted. But they do NOT show a parallel to BM behavior. Actually, I
know that Mario is aware of these figures!

Here we see that at one single auditory nerve fiber recording site the
firing rate was strongest at the lowest sound levels of 40 and 50 dB, when
the probe tones had a frequency 6 kHz. At a sound level of 100 dB the firing
rate was *again* strongest, when the probe tones had a frequency 6 kHz. Most
interestingly, the data for the 100 dB probe tones show a second peak at 5.2
kHz. So, the neural data mainly reflect the OHC tuning, and secondarily also
reflect the passive BM tuning. At following stages of neural processing
these secondary peaks are then filtered out by lateral inhibition.

4) Conclusion:
The dissociation of BM tuning and auditory nerve fiber tuning is perfect and
obvious, and it has a physiological explanation. It is one of the many
pieces of evidence that demonstrate the *real* function of BM mechanics.

I hope these data can be of use. I would
appreciate if Richard F. Lyon, or somebody else
from the list, could check the referenced data
and confirm for the list that they are real.

I haven't been able to find the evidence for the assertion that neural
tuning remains unchanged in some way that differs from the sense in which
BM tuning remains unchanged;

[.......]

Where should I look to study the data on this idea that underlies your
modeling approach?

You can take any data on level dependence of neural responses. None of
them mirrors the half-octave shift of BM tuning.

I've looked at lots of data, and you must be interpreting it differently
from how I am. So if you have something specific that we can look at and
discuss, we can try to resolve that difference. Lacking that, I'll stick
with Ruggero's interpretation that says mechanical and neural are
essentially the same, not different.